The invention is in the field of magnetic resonance imaging (MRI) and pertains to the use of pulse sequences which seek to optimize trade-offs between parameters such as data acquisition time, signal-to-noise ratio (SNR), and image resolution. Two such pulse sequences are known under the names RARE and GRASE and are discussed in Feinberg D A, Kiefer B, and Litt A W. High Resolution GRASE MRI of the Brain and Spine: 512 and 1024 Matrix Imaging. J Compu Assist Tomogr 1995; 19(1): 1-7. The cited article is hereby incorporated by reference.
Spin echo trains produced with the Carr-Purcell-Gill-Meiboom (CPMG) pulse sequence can be useful for MRI. For example, as illustrated in FIG. 1 and discussed in Mansfield P and Pykett E L. Biological and Medical Imaging by NMR. J Magn. Resonance 29, 355-373 (1978), an encoding methodology can accumulate phase shifts in successive signals by reversing the polarity of a constant absolute value amplitude gradient after each RF pulse. As seen in FIG. 1, a 90.degree. radio frequency (RF) excitation signal is followed by a sequence of 180.degree. rephasing RF pulses which are equally spaced in time from each other. A slice select gradient pulse Gs is active at times which coincide with each of the RF signals. A read gradient Gr is pulsed to be active during each interval between two successive 180.degree. RF signals. A phase encode gradient Gp is pulsed to be active for each readout gradient pulse Gr (except for the read gradient which is active at time ko which corresponds to zero phase encoding). The Gp gradient pulses are centered relative to, and are longer and have lower amplitudes than, the Gr gradient pulses. In addition, the Gp gradient pulses alternate in sign but maintain the same absolute value amplitude and the same time duration (and, therefore, have a constant area). An MRI signal is sampled during each read gradient pulse Gr, to derive digital samples of the analog MRI signals illustrated at k.sub.0, k.sub.1, k.sub.2, k.sub.3, k.sub.4, etc. These digitized MRI signals are stored in K-space in lines which are parallel to a frequency axis and are spaced from each other along a phase axis. The MRI signal for k.sub.0 is stored at an origin of the phase axis. The lines typically conform to a square or rectangular area in K-space. An MRI image can be derived from the K-space matrix using, for example, known two-dimensional Fourier Transform (2D FT) image reconstruction techniques. While this method makes efficient use of data acquisition time because it uses Gr and Gp gradient pulses that overlap in time, it will typically create image artifacts because stimulated echoes and spin echoes have different magnetization pathways and stimulated echoes can be undesirably phase encoded. The stimulated echo magnetization can be stored in the longitudinal plane during some of the time intervals between successive 180.degree. RF signals and may not experience the same number of switched phase encode gradient pulses as do the spin echoes, and their unequal phase can cause destructive interference in the net MRI signal.
One way to deal with stimulated echo artifacts involves phase rewind phase encoding as discussed in U.S. Pat. No. 4,697,148. Phase rewind encoding involves inserting a phase rewind gradient pulse to cancel the effect of the immediately preceding phase encoding gradient pulse. As a result, after each 180.degree. rephasing RF signal, the spin echoes and the stimulated echoes both should have zero phase and should be subjected to a phase encode gradient pulse which concurrently and equally would encode them both so as to avoid phase incoherence.
As illustrated in FIG. 2, this phase rewind approach can be applied in CPMG pulse sequences. In FIG. 2, the RF signals and the Gr gradient pulses are as in FIG. 1, but the phase gradient pulses Gp now comprise a sequence of alternating phase encode and rewind gradient pulses in which each phase encode pulse is positive and precedes a read gradient pulse Gr which is followed by a phase rewind gradient pulse which is negative. The magnitudes (absolute values) of the phase gradient pulses decrease from the start toward the middle of the sequence, as illustrated in FIG. 2, and increase after the midpoint at k.sub.0 in the sequence, where the phase encoding is zero (not shown in FIG. 2). One example of such phase rewind methodology for a fast CPMG is known as the RARE pulse sequence (also known as Turbo SE and FAST SE), as discussed in U.S. Pat. No. 4,818,940 (which refers to changing the intensity and/or duration of the phase encoding gradient after every 180.degree. pulse), and a more recent example is the gradient and spin echo sequence known as GRASE.